How to implement and extend Joshua's builder pattern in .net?

Question

• How can we implement the Builder pattern of Joshua\'s Effective Java in C#?

Below is the code I have tried, is there a better way to do this?

Answer 1

In Protocol Buffers, we implement the builder pattern like this (vastly simplified):

public sealed class SomeMessage
{
  public string Name { get; private set; }
  public int Age { get; private set; }

  // Can only be called in this class and nested types
  private SomeMessage() {}

  public sealed class Builder
  {
    private SomeMessage message = new SomeMessage();

    public string Name
    {
      get { return message.Name; }
      set { message.Name = value; }
    }

    public int Age
    {
      get { return message.Age; }
      set { message.Age = value; }
    }

    public SomeMessage Build()
    {
      // Check for optional fields etc here
      SomeMessage ret = message;
      message = null; // Builder is invalid after this
      return ret;
    }
  }
}

This isn't quite the same as the pattern in EJ2, but:

• No data copying is required at build time. In other words, while you're setting the properties, you're doing so on the real object - you just can't see it yet. This is similar to what StringBuilder does.
• The builder becomes invalid after calling Build() to guarantee immutability. This unfortunately means it can't be used as a sort of "prototype" in the way that the EJ2 version can.
• We use properties instead of getters and setters, for the most part - which fits in well with C# 3's object initializers.
• We do also provide setters returning this for the sake of pre-C#3 users.

I haven't really looked into inheritance with the builder pattern - it's not supported in Protocol Buffers anyway. I suspect it's quite tricky.